1. Field of the Invention
The present invention relates to techniques facilitating remote copying between different disk systems.
2. Description of the Related Art
The specification of European Patent Publication No. 0672985 discloses writing techniques of a remote copy operation effected between disk systems at remote sites. According to the specification of European Patent Publication No. 0672985, a primary disk system, receiving a write request directly from a computer (hereinafter called a primary CPU) executing an application, reports a write request completion to the primary CPU immediately after the write request process. Further, a copy of the write request received at the primary disk system is read by the primary CPU. A write request issued from the primary CPU includes a time when the primary CPU issued the write request. When the copy of the write request is read by the primary CPU, the write time is also passed to the primary CPU. The read copy of the write request is sent from the primary CPU to a computer (hereinafter called a secondary CPU) at a remote site.
The secondary CPU receiving the copy of the write request and the write time writes the write data in the write request in the order of write time, in a secondary disk system connected to the secondary CPU.
According to the invention described in the specification of European Patent Publication No. 0672985, it is possible to avoid or prevent leaving intermediate results of transactions generally used in an on-line system.
For example, not to leave intermediate results means to preclude a state or condition in which data integrity is not guaranteed between a system A and a system B, for example, when the system A executes a transaction of transmitting data to the system B, although the system A transmitted the data, the data is not written in the system B. Generally, in the on-line system, since the unit of data recovery is a transaction, leaving the intermediate results of a transaction is a very serious obstacle.
Next, a brief description will be made on the fact that if processes described in the specification of European Patent Publication No. 0672985 are executed, it is possible to avoid leaving the intermediate results of a transaction. The primary and secondary disk systems effecting remote copying have a disk for storing a database such as account information and a disk for storing a journal of an update history of each transaction.
When the primary CPU recovers after it crashes, a recovery process program at the primary CPU analyzes the journal in the primary CPU and executes processes such as a process of recovering the state before the start of transaction execution for the update results of an uncompleted transaction by using the journal, so that the intermediate results of the transaction are prevented.
Further, data written in the secondary disk system becomes valid, for example, when the primary disk system storing the latest data is broken. It is guaranteed that although the secondary disk system does not store the latest data, the write data in the write requests issued before a certain time are already written. Therefore, the intermediate results of a transaction can be prevented, if the secondary CPU executes a process similar to the recovery process which is executed when the primary CPU recovers after it crashes, by using the database and journal stored in the secondary disk system.